STRUCTURAL GEOLOGY AND GEOCHRONOLOGY OF THE SHORES MÉLANGE: POLYPHASE DEFORMATION ACROSS THE TACONIAN SUTURE IN THE PIEDMONT, VIRGINIA

In the central Virginia Piedmont, the Shores Mélange is postulated to be the suture between Laurentia and the exotic (peri-Gondwanan) Chopawamsic Terrane, an Ordovician volcanic arc. The Shores Mélange contains a sequence of metasedimentary and mafic metavolcanic rocks. In the west, the bedrock is primarily micaceous meta-graywacke with thin quartzite layers. Layered, epidote-rich greenstone/schist separates the meta-graywacke from a coarse-grained schistose gneiss with mafic/ultramafic blocks (≤2 m). The schistose gneiss is intruded by a suite of thin (<10 m) leucogranite dikes that cross-cuts an older foliation in the metasedimentary rocks. To the east, Shores rocks are in steep contact with mid-Ordovician Chopawamsic felsic metavolcanic and metasedimentary rocks. In the mélange, both the country rock and dikes were subsequently folded during polyphase NW-SE shortening and dextral shearing under greenschist facies conditions.

Here we present new zircon U-Pb geochronology to determine the maximum depositional age (MDA) of metasedimentary rock in the mélange, the emplacement age of leucogranite dikes, and to delineate the later metamorphic and tectonic history of this terrane. DZ from the schistose gneiss constrain the MDA of these metasedimentary rocks to be ~460 Ma, older age populations in these samples are dominated by non-Laurentian Grenville-age zircons. Leucogranites are characterized by high Si and low Zr, and likely formed as minimum melts. Most zircons are xenocrystic with Grenville ages (1.0 – 1.2 Ga), but rare magmatic zircons suggest these dikes intruded during the mid-Paleozoic.

Based on regional geological considerations and local structure, we infer that the Shores Mélange formed as a late Ordovician accretionary wedge that incorporated blocks of ocean-floor igneous rocks and detritus from the Chopawamsic arc during Taconian accretion. Post-suture leucogranite dikes were sourced from partial melting of Laurentian continental basement that impinged upon the Taconian subduction channel and effectively ended the Taconian orogeny. Rocks in the Shores Mélange were modified by later Alleghanian greenschist facies metamorphism and pure shear-dominated transpressional deformation.